@Amy - thanks for the tip! I can't find it either, but nice to have a starting point.
I've found the Maje Licorne top on their Facebook photos though. Very similar!
Update: it's definitely the Maje Licorne (floaty sleeveless top) but I can't find it in the same colour-way as Alex's.
It did accidentally lead me to find another of Alex's top's from The One Show (23 June 2016) that I also loved - the Louise Crocheted Cotton Tank by Maje. £159, reduced to £79.50 but now, sadly, sold out.